Rotation prevention mechanism for preventing rotation of the revolving scroll and defining the radius of revolution thereof such as a crank mechanism and Oldham coupling has been adopted in scroll fluid machines.
First, the principle of scroll compressor will be explained briefly with reference to FIGS. 8a to 8d. 
A scroll compressor consists of a stationary scroll having a spiraling scroll lap 011 and a revolving scroll having a spiral lap 013. Gas ingested from an inlet port 017 is compressed as a revolving scroll revolves and the compressed gas is discharged from a discharge port 025 at the center. A stationary scroll lap 011 is formed on a disk fixed perpendicular to a rotation shaft. The revolving scroll lap 013 and the stationary scroll lap 011 spiral with phase difference of 180°. A crescent-shaped enclosed space (compression room) 015 formed between the inside surface 011b of the stationary scroll lap 011 and the outside surface 013a of the revolving scroll laps 013 is conveyed to the center of the scrolls reducing gradually in volume as the revolving scroll revolves (orbits).
In FIG. 8a, suction process ends when gas ingested from the suction port 017 is enclosed in the compression room formed between the outside surface 013a of the revolving scroll laps 013 and the inside surface 011b of the stationary scroll lap 011. Then, when a rotation shaft having an offset pin by which the revolving scroll is supported further rotates 90° as shown in FIG. 8b, the gas in the compression room 015 is conveyed toward the center of the scrolls and decreased in volume as compared with the compression room 015 in FIG. 8a. 
When the rotation shaft further rotates 90° as shown in FIG. 8c, the gas in the compression room 015 is further conveyed toward the center and further decreased in volume.
In FIG. 8d, the compression room 015 is communicated with the discharge port 025 at the center and the compressed gas is discharger from the discharge port 025 as the rotation shaft further rotates.
As describer above, the revolving scroll must be orbited about the center of the rotation shaft without rotation. For allowing the revolving scroll to orbit without rotation, the revolving scroll is connected to the rotation shaft via an Oldham coupling or crank mechanism.
The principle of Oldham coupling will be briefly explained referring to FIG. 9. The Oldham coupling is a shaft coupling which can transmit torque between two parallel shafts offset from each other. In FIG. 9, a drive shaft 038 is supported for rotation about a rotation axis C1 and a driven shaft 039 is supported for rotation about a rotation axis C2 which is offset from the rotation axis C1 by E. The drive shaft 038 and driven shaft 039 have a drive flange 034 and driven flange 036 respectively. A disk 031 has a rectangular protrusion 032 and 033 formed on both sides thereof respectively, both the protrusions 032 and 033 extending perpendicular to each other passing through the center of rotation of the drive shaft 038. The drive flange 034 has a straight groove 035 and the driven flange 036 has a straight groove 037. The protrusion 032 of the disk 031 is received in the groove 035 of the drive flange 034 and protrusion 033 of the disk 031 is received in the groove 037 of the drive flange 034. When the drive shaft 038 is rotated, the driven shaft 039 is rotated in the same direction at the same rotation speed.
When the drive shaft is fixated not to be rotated and a member 040 supporting the driven shaft 039 is revolved about the rotation axis C1, the driven flange 036 revolves about the rotation axis C1 without itself being rotated, for its rotation is prevented by the engagement of the rectangular protrusions 032, 033 with the grooves 035, 036, the member 040 rotates relative to the drive shaft 039 instead.
In a case of scroll compressor, the drive flange 034 is a stationary scroll, the driven flange 036 is a revolving scroll, and the member 040 is a crank portion of a crankshaft for driving the compressor. Usually, said member 040 is formed to be a crank pin to be received via a bearing in a center hole of the revolving scroll, and said rectangular protrusions and grooves are formed on peripheral portions of the disk 031 (Oldham ring), drive flange 034 (stationary scroll), and driven flange 36 (revolving scroll) respectively.
For example, an Oldham coupling is adopted in scroll fluid machine disclosed in Japanese Patent No. 2756808 (patent literature 1). The scroll compressor is shown in longitudinal sectional view in FIG. 10a. A stationary scroll 051 having a spiraling lap 050 is fixed to a casing 052. A revolving scroll 054 having a spiral lap 053 is supported via a bearing 058 by a crank pin 056 of a crankshaft 057 supported for rotation by the casing 052. Oldham ring 059 is provided between the stationary scroll 051 and revolving scroll 054. When the crankshaft 057 is rotated, the revolving scroll 054 orbits around the rotation axis of the crankshaft without rotation.
The Oldham ring 059 has, as shown in FIG. 10b, rectangular protrusions 063 on one side thereof and rectangular protrusions 064 on the other side thereof. The protrusions 063, 064 are made by piling carbon fiber and cementing them by resin, to have improved anti-wear property.
In Japanese Laid-Open Patent Application No. 2003-106268 is disclosed a scroll fluid machine which adopts pin-crank type anti-rotation devices. As shown in FIGS. 11a, 11b, compression rooms 072 are formed between the spiral laps of the stationary scroll 070 and revolving scroll 071, and the revolving scroll 071 is supported by an offset pin portion of a crankshaft 073 via bearings 074.
Three pin crank type anti-rotation mechanism 079 are provided along a circle at equal circumferential spacing such that a journal of a pin crank 078 is supported by a casing, to which the stationary scroll 070 is fixed and the crank shaft 073 is supported for rotation, via two rolling bearings 077 and 077, and an offset pin portion of the pin crank 078 is supported by the end plate of the revolving scroll 071 via a rolling bearing 075.
In an Oldham coupling type anti-rotation mechanism, grooves and rectangular protrusions to be received in the grooves are formed as shown in FIG. 9, so abrasion of the grooves and rectangular protrusions tend to occur resulting in increased clearance therebetween, which produces vibration and noise. Therefore, according to the patent literature 1, the Oldham coupling type anti-rotation mechanism is composed to be improved in anti-wear property.
In a scroll fluid machine adopting pin crank type anti-rotation mechanism as shown in FIGS. 11a, 11b, usually three pin cranks are provided, and angular contact ball bearings are used to maintain proper clearance between the top faces of the scroll laps and the mating mirror surfaces of the stationary and revolving scrolls, structure becomes complicated resulting in increased manufacturing cost.
Further, the bearings of the pin cranks must be lubricated by lubrication oil or grease, controlling of temperature of the bearings is necessary, and there remains a problem that noise increases due to wear of the bearings.
In either case of adopting as anti-rotation mechanism the Oldham coupling mechanism or pin crank mechanism, it is necessary to supply lubrication oil and take measure against abrasion, so it is difficult to provide an oil-free scroll fluid machine. Even if the anti-rotation mechanism is composed of self-lubricating material, to completely solve the problem of increase in clearances is difficult as long as sliding parts exist in the mechanism.
Even if oil-free construction is realized in the compression rooms formed by the scroll laps, there remains fear that lubrication oil or grease for lubricating the anti-rotation mechanism intrudes into the compression rooms of the scroll compressor.